I've measured up to 9dB gain horn loading a smallish cone driver, but some of this was lying; it was energy that would have gone elsewhere in the anechoic chamber, but was now hitting the microphone. And I was measuring for a given power input, not a voltage, and the impedance goes up on a properly loaded driver. Still, some was real, due to improved air coupling.

But this would depend on the driver; a high resonance, light cone, short throw, focused magnetic gap - a driver designed for horns – will give more gain than a general purpose driver, and lots more than one designed to go as low as possible, long throw, soft suspension and mass-loaded cone to bring the resonance down.